The present invention relates to medical devices and methods. More specifically, the present invention relates to intervertebral disc prostheses.
Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.
One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.
Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.
When one or more damaged intervertebral discs cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without fusion of the two vertebrae adjacent to the disc. Fusion of the two vertebrae is achieved by inserting bone graft material between the two vertebrae such that the two vertebrae and the graft material grow together. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and graft material in place while they permanently fuse together. Although fusion often treats the back pain, it reduces the patient's ability to move, because the back cannot bend or twist at the fused area. In addition, fusion increases stresses at adjacent levels of the spine, potentially accelerating degeneration of these discs.
In an attempt to treat disc related pain without fusion, an alternative approach has been developed, in which a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) is inserted between two vertebrae. A number of different artificial intervertebral discs are currently being developed. For example, U.S. Patent Application Publication Nos. 2005-0021146, 2005-0021145, and 2006-0025862, which are hereby incorporated by reference in their entirety, describe artificial intervertebral discs. This type of intervertebral disc has upper and lower plates positioned against the vertebrae and a mobile core positioned between two plates to allow articulating, lateral and rotational motion between the vertebrae.
Another example of an intervertebral disc prostheses having a movable core is the CHARITE artificial disc (provided by DePuy Spine, Inc.) and described in U.S. Pat. No. 5,401,269. Other examples of intervertebral disc prostheses include MOBIDISC (provided by LDR Medical), the BRYAN Cervical Disc (provided by Medtronic Sofamor Danek, Inc.), and the PRODISC (from Synthes Stratec, Inc.) and described in U.S. Pat. No. 6,936,071. Some of these intervertebral discs are mobile core discs while others have a ball and socket type two piece design. Although existing disc prostheses provide advantages over traditional treatment methods, improvements are ongoing.
These known artificial intervertebral discs generally include upper and lower plates which locate against and engage the adjacent vertebral bodies, and a core for providing motion between the plates. The core may be movable or fixed, metallic, ceramic or polymer and generally has at least one convex outer surface which mates with a concave recess on one of the plates in a fixed core device or both of the plates for a movable core device. In order to implant these intervertebral discs, the natural disc is removed and the vertebrae are distracted or forced apart in order to fit the artificial disc in place. Depending on the size of the disc space, many of the known artificial discs have a height which is higher than desired resulting in an unnaturally over distracted condition upon implantation of the disc. For example, a smallest height disc available can be about 10-13 mm for lumbar discs and about 5-6 mm for cervical discs.
Currently available artificial intervertebral discs do not provide a desired low profile for some patients with smaller disc heights. The use of an artificial disc which is too large for a patient's disc space can result in limited mobility of the disc. An improperly sized artificial disc can also move out of place in the disc space. Accordingly, it would be desirable to provide a lower height disc which mimics more closely the natural anatomy for smaller patients.
In addition, the vertebral body contacting surfaces of many of the known artificial discs are flat. This flat configuration does not generally match the surfaces of the vertebral bodies resulting in less than ideal bone to implant contact surfaces. It would be desirable to provide a more anatomically shaped vertebral body contacting surface for an artificial disc.
Therefore, a need exists for an improved artificial intervertebral disc with a lower disc height and more anatomically shaped upper surface.